1. Field of the Invention
This invention relates to surfactant-enhanced oil recovery methods and compositions used therein wherein the surfactants employed are anionic surfactants. More particularly, this invention relates to the use of certain alkyl-substituted aryl sulfonates in enhanced oil recovery processes wherein the aryl moiety contains two aromatic rings, such as, for example, biphenyl, diarylalkane, or naphthalene groups which are substituted by at least two alkyl groups and at least two functional groups, as defined below, and wherein at least one of said functional groups is a sulfonate. These surfactants are particularly effective in conjunction with the use of steam in the enhanced recovery of subterranean oil, especially where there are present high aqueous concentrations of salts in the oil reservoirs. These surfactants, it has been found, are highly salt-tolerant and maintain their effectiveness even in the presence of divalent salts.
Many different classes of organic compounds are known to have surfactant properties, and a wide variety of these surfactants have been used for enhanced oil recovery. Since in most oil reservoirs there are found considerable amounts of water, and since this conate water frequently contains sizeable amounts of various salts dissolved in it, a very important property of any useful surfactant will be its ability to mobilize oil in saline environments. Anionic surfactant systems, including most alkylaromatic sulfonates, are excellent surfactants, but only at low salt concentrations. In the presence of dissolved salts, the hydrophilic portion of these molecules become less soluble because of the increased competition for water hydration molecules from the salt ions. Hence, as salinity increases surfactant solubility tends to decrease. The effect may be even more pronounced for salt solutions containing divalent cations. Therefore, it is an object of this invention to provide alkylaromatic sulfonates having tolerance for high concentrations of salts, particularly salts containing divalent cations, preferably for use in tertiary recovery systems employing steam rather than water.
2. Prior Art
Numerous patents address aqueous anionic surfactant systems for use in the secondary and tertiary recovery of oil from subterranean reservoirs. See, for example, U.S. Pat. Nos. 1,981,337; 3,469,630; 3,508,612; 3,811,504; 3,811,505; 3,811,507; or 3,945,437. See also, regarding arylsulfonates compounds per se, U.S. Pat. Nos. Re: 22,548; 2,368,361; 3,361,313; 4,172,029; as well as Okhmedov et al, Chem Abstr. 59, 15203H (1960); Moriga, Chem. Abstr. 70, 5421 N (1968); and Golikov et al, Chem Abstr. 76, 80291 c (1971).
In addition, Canadian Patent, No. 1,164,889 (1984) teaches the use of alkyl biphenyl disulfonates in the enhanced recovery of underground oil. However, this invention teaches (1) that preferably the biphenyl compounds be used as cosurfactants with petroleum sulfonates; (2) that the biphenyl compounds be at least 90% disulfonated; and (3) that the alkyl substituents be desirably monoalkyl with only a small amount of dialkyl compounds mixed in, wherein the alkyl group contains at least 6 carbon atoms and preferably 10-18 carbon atoms. As will be apparent from the following description of the present invention, the compounds of the Canadian patent and the compositions in which they are used in oil recovery methods differ in important ways from those claimed hereinbelow. Other differences between the other prior art compounds and methods and those claimed herein will likewise be evident from the following description.